I've had some disappointments, but unfortunately it took me a while to realize that the guns sucked. I'm going to cause some butthurt with this, but with this thread topic it's inevitable.
S&W TDAs. My 669 and 1006 looked so awesome, and they were my first centerfire autos. But after I shot them for a year, they turned out to be steaming piles of shit, with trigger problems, frame peening, firing pin issues, and an incredibly stupid number of parts. When got finally an HK USP, I realized that the S&W autos were guns that looked good, but weren't well designed.
Wait for it...
1911s. After being a happy HK owner, I started watching the Unit, and got sucked into the idea that if I shot 1911s then I could be Operator as Fuck. Fast forward a few years, thousands of dollars, and countless mechanical issues including special magazines that degrade over time, special ammo, tuned extractors that are ammo-specific, fitted safeties, a $2500 gun with frame permanently damaged by peening of the slide stop, the flimsy wedge of the ambi safety broke, grip safety sticking, malfunctions that take minutes to never to resolve, and more. I started to realize that these were not guns I wanted as life safety equipment, and I wasn't interested in owning them anymore.
Then I shot a Glock. And holy shit! I shot it as well as I shot my 1911s. Maybe not as tight a group at 25 yds, but in every way I cared about, the Glock was equal to or better than the 1911s. I sold all the 1911s and never looked back. (Except for a brief try shooting USPSA Open Division with a STI 2011, that is now also sold with no regrets.)
Sig p320. This was in some ways a good gun, except for some serious design flaws, including firing when dropped on carpet from 24". I sold them all, at a significant loss.